However, nobody would have predicted the team would still be operating both rovers in 2009.

Ed Weiler, associate administrator for NASA's Science Mission Directorate at NASA Headquarters in Washington, said the American taxpayers were initially told each rover had a prime mission plan of three months.

"The twins have worked almost 20 times that long. That's an extraordinary return of investment in these challenging budgetary times."

Spirit and Odyssey have pioneered numerous important discoveries about wet and violent environments on ancient Mars as well as returning a quarter-million images.

Together, they have driven more than 13 miles, climbed a mountain, descended into craters, struggled with sand traps and aging hardware, survived dust storms, and relayed more than 36 gigabytes of data via NASA's Mars Odyssey orbiter. The rovers continue to remain operational for new campaigns the team has planned for them.

"These rovers are incredibly resilient considering the extreme environment the hardware experiences every day," said John Callas, JPL project manager for Spirit and Opportunity. "We realize that a major rover component on either vehicle could fail at any time and end a mission with no advance notice, but on the other hand, we could accomplish the equivalent duration of four more prime missions on each rover in the year ahead."

The vehicles' longevity has received plenty of aid from the Martian wind, which has blown dust from the rovers' solar panels over the years, but it's been 18 months since Spirit has had a good cleaning. Dust-coated solar panels barely provided enough power for Spirit to survive its third southern-hemisphere winter, which ended in December.

Callas said this last winter was a squeaker for Spirit. "We just made it through," he added.

During this spring and summer, the team plans to drive Spirit to a pair of destinations about 200 yards south of the site where it spent most of 2008. One is a mound that might yield support for an interpretation that a plateau Spirit has studied since 2006, called Home Plate"”a remnant of a once more-extensive sheet of explosive volcanic material. The other destination is a house-size pit called Goddard.

But Steve Squyres of Cornell University, in Ithaca, N.Y. said Goddard doesn't look like an impact crater. "We suspect it might be a volcanic explosion crater, and that's something we haven't seen before," said Squyres, the principal investigator for the rover science instruments.

In fact, the light-toned ring around the inside of the pit might add information about a nearby patch of bright, silica-rich soil that Squyres counts as Spirit's most important discovery so far. Spirit churned up the silica in mid-2007 with an immobile wheel that the rover has dragged like an anchor since it quit working in 2006. The silica was likely produced in an environment of hot springs or steam vents.

The Endeavour Crater will soon be Opportunity's next destination. It is approximately 14 miles in diameter, more than 20 times larger than another impact crater, Victoria, where Opportunity spent most of the past two years. The rover is facing a route containing major obstacles while on its way to the crater.

The rover has already started its 7-mile route toward Endeavour and has also stopped to inspect the first of several loose rocks the team plans to examine along the way. NASA is plotting its route using high-resolution images from the Mars Reconnaissance Orbiter, which reached Mars in 2006, to avoid potential sand traps that were not previously discernable from orbit.

Squyres said the journeys have turned into humanity's first overland expedition on another planet.

"When people look back on this period of Mars exploration decades from now, Spirit and Opportunity may be considered most significant not for the science they accomplished, but for the first time we truly went exploring across the surface of Mars," he said.